The present invention relates generally to circuits used in communication receivers, and more particularly, to differential circuits with low even order distortion and low DC offset for use in a communication receiver.
A differential circuit operates on the difference between two input signals. Its structure couples together two half-circuits and its performance largely depends on the symmetry between these two half-circuits.
The linearity performance of differential circuits is critical in certain applications, such as in communication receivers. Oftentimes, communication receivers must process very strong unwanted or interfering signals without corrupting the wanted or desired communication signal. In many applications, the desired communication signal can be extremely small. It would therefore be advantageous to have differential circuits with improved linearity and low distortion.
The present invention includes a differential feedback system that minimizes even order distortion of a differential circuit. The feedback system controls key aspects of the differential circuit, improving its symmetryxe2x80x94the key to the differential circuit""s ability to reject even order distortion and dc offsetsxe2x80x94and thereby extending its linearity. In one embodiment, a feedback circuit resolves even order distortion and dc offsets by analyzing the mean value of the differential circuit""s output waveform, which is typically zero for most modulated signals.
In one embodiment of the present invention, a feedback circuit is provided for use with a differential circuit to reduce even-order distortion and dc offset of a difference output signal produced by the differential circuit. The feedback circuit includes an integrator coupled to receive the difference output signal from the differential circuit and produce an integrator output signal. The feedback circuit also includes a control circuit coupled to the integrator to receive the integrator output signal to produce a control signal that is coupled to the differential circuit, wherein the control signal controls the differential circuit to reduce the even-order distortion and the DC offset.
In another embodiment of the present invention, a method is provided for reducing even-order distortion and dc offset of a difference output signal produced by a differential circuit. The method includes steps of integrating the difference output signal to produce an integrated signal, translating the integrated signal to produce a control signal, and controlling the differential circuit with the control signal.